1. Field of the Invention
The invention relates to a combustion state estimation method that estimate the length of a first combustion period (first combustion period length) regarding combustion of fuel in an internal combustion engine as one of parameters that indicate a combustion state.
2. Description of Related Art
Various techniques for estimating changes in the amount (heat release amount, heat generation amount) Q of heat that is generated as a result of combustion of fuel in a combustion chamber have been suggested in order to estimate a combustion state of fuel in an internal combustion engine. Particularly, changes in heat release rate (dQ/dθ) in a crank angle (that is, a heat release rate waveform) are important in estimating a combustion state of the internal combustion engine. The heat release rate dQ/dθ is defined as a heat release amount [J/CA] per unit crank angle with respect to rotation of a crankshaft.
The Wiebe function is a function that defines the relationship between various variables and a heat release rate, and is well known as one of effective functions for estimating a heat release rate. Various variables that are used in the Wiebe function are, for example, a shape parameter, an efficiency, an ignition delay period, and the like. Japanese Patent Application Publication No. 2007-177654 (JP 2007-177654 A) describes a technique for determining these variables on the basis of parameters that indicate an engine operating state (for example, spark timing, engine rotation speed, load factor, air-fuel ratio, and the like).
JP 2007-177654 A further describes that it is possible to estimate a first combustion period on the basis of the Wiebe function. The first combustion period is a period from the ignition timing of fuel in a spark-ignition internal combustion engine to timing at which the release rate of heat (heat release rate dQ/dθ) resulting from combustion of the fuel is maximum (that is, maximum heat release rate timing). The length (crank angle width) of the first combustion period is also referred to as first combustion period length. The first combustion period length is one of important parameters in estimating the combustion state of the internal combustion engine.
However, the first combustion period length significantly changes in response to the valve opening characteristic of an intake valve (for example, a characteristic value regarding a period during which the intake valve is open, which is determined by intake valve phase angle, intake valve operating angle, intake valve opening timing, intake valve closing timing, and the like (described later). This is inferably due to the fact that the strength of a disturbance of air stream in a cylinder (combustion chamber) changes when the valve opening characteristic of the intake valve is changed and, as a result, combustion speed changes.
Therefore, if the first combustion period length is estimated by utilizing the Wiebe function, but when the valve opening characteristic of the intake valve changes, it is required to determine the variables of the Wiebe function for various engine operating state parameters again. In addition, when the first combustion period length is acquired with actual measurement, an enormous amount of experiments need to be carried out for various engine operating states each time the valve opening characteristic of the intake valve is changed. In addition, even when the first combustion period length is estimated by the use of a model suggested by the applicant (model expression) described in this specification, it is similarly required to identify various parameters again each time the valve opening characteristic of the intake valve changes. That is, it is required to expend extremely large amounts of time and effort in order to accurately acquire a first combustion period length for each of various valve opening characteristics of the intake valve.